1. Field of the Invention
The present invention relates to magnesium oxide (MgO) powder adapted for use in phosphate cements which provide rapid-setting compositions, and more particularly to cements containing magnesium oxide powder adapted to be used in hardenable phosphate compositions which set rapidly after a lead time of sufficient length to allow the composition to be worked, placed and finished.
2. Description of the Prior Art
Cements are particulate materials that set up and harden to strong, dense monolithic solids upon being mixed with a liquid and allowed to stand. Cements find utility in many areas of commerce, especially in the construction industry, where they are often used in conjunction with various aggregate materials to form such articles as concrete slabs, panels and pillars, masonry binders and floorings. Rapid-setting cements are advantageous not only in new but especially in rehabilitative construction applications such as road patching, resurfacing floors, and repairing masonry.
The technology relating to quick-setting inorganic cements has been focused largely on phosphate systems in which a basic component, usually magnesium oxide, reacts with phosphoric acid or a salt thereof in the presence of water. One of the problems encountered with this system is that such compositions set too rapidly to enable the user to comfortably place them by pumping, pouring, trowelling, etc.
U.S. Pat. No. 4,394,174 describes cement products and rapid-setting compositions made therefrom, containing magnesium oxide and aluminum phosphate, which compositions are revealed to be workable for a length of time, i.e., at least ten minutes, sufficient, for example, to allow them to be comfortably mixed and finished. Thereafter the compositions are said to harden quickly, e.g., within one hour, at which time, for example, vehicular traffic can be allowed to pass over a road repair patch. The required prolonged working time of these compositions is achieved by retarding their set time by controlling the concentration, particle size, and the surface area of the magnesium oxide in a manner such that the peak of the exotherm is reached after about a ten-minute interval.
U.S. Pat. No. 4,152,167 describes a dry matrix-forming composition containing magnesium oxide, ammonium phosphate, and a polyphosphate which can be admixed with aggregate or filler material to provide a product that sets to form a solidified mass when mixed with water. The composition, when mixed with water, is said to be readily workable prior to setting, although the exemplified mixtures remained fluid for only 1 to 8 minutes.
U.S. Pat. No. 3,879,209 describes a process for making quick-setting cements based on magnesium oxide and a solution containing ammonium polyphosphates. These cements are said to "set hard" within a few minutes, generally about 45 minutes.
It has been found that magnesium oxide/phosphate rapid-setting compositions of the type described above may be inconsistent in their working times if a low-purity magnesium oxide, e.g., dead-burned magnesite, is present therein. Considerable variations in working time have been observed in compositions made with a low-purity dead-burned magnesite containing about 87% magnesium oxide when the magnesite, MgCO.sub.3, although consistently of the same grade designation and particle size distribution, has been received in different shipments from the same supplier. Such variations have been encountered even in compositions in which the concentrations of the components and the surface area and particle size of the magnesium oxide have been carefully controlled to provide acceptable working times. As a result, in many instances the working time is too short. Inasmuch as it is economically advantageous to use low-purity magnesium oxide in phosphate cements, a means is needed for providing a consistent adequate working time in rapid-setting compositions made from phosphate cements containing low-purity magnesium oxide.